System to reduce interface emulsion layer formation in an electrostatic dehydrator or desalter vessel through use of a low voltage electrostatic interface emulsion treatment system inside the vessel

Abstract

A system for separating the components of an incoming oil-water mixture includes two electrode sets, one set arranged to apply an electrostatic field to an oil layer residing within a separator vessel and the other set arranged to apply an electrostatic field to the interface emulsion layer residing within the separator vessel. The first set of electrodes is in communication with a high voltage power source that ranges from 1 to 60 kV; the second set of electrodes is in communication with a low voltage power source that is no greater than 5 kV. Each set of electrodes may also be in communication with a second voltage source to provide increased power to promote effective coalescence. The system may also include power electronics to produce a variable amplitude and a variable frequency voltage supply to one or both electrode sets.

Claims

1. A system for reducing interface emulsion layer formation when separating the components of an incoming oil-water mixture, the system comprising: a separator vessel including an oil/water interface location including a water/emulsion boundary location; a first set of vertically oriented electrodes located in an upper half of the vessel and connected to a high voltage power source greater than 5 kV; a second set of vertically oriented electrodes located entirely below the first set and above the water/emulsion boundary and connected to a low voltage power source in a range of 1 kV to 5 kV, a portion of the second set being within the oil/water interface location.

2. A separator vessel for reducing interface emulsion layer formation when separating the components of an oil-water mixture, the vessel comprising: an oil layer volume; an oil/water interface location; a first set of vertically oriented electrodes located in the oil layer volume and connected to a high voltage power source; and a second set of vertically oriented electrodes connected to a low voltage power source in a range of 1 kV to 5 kV, the second set located entirely below the first set and within the oil/water interface location.

3. A separator vessel according to claim 2 further comprising power electronics located external to the separator vessel, the power electronics in communication with at least one of said voltage power sources to produce a variable amplitude and a variable frequency voltage supply.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view of a prior art dehydrator or desalter vessel having a high voltage electrode grid in the oil layer of the vessel.

(2) FIG. 2 is a view of a preferred embodiment of a dehydrator or desalter vessel having a high voltage electrode grid in the oil layer of the vessel and a low voltage electrode grid at the oil/water interface and in the interface emulsion or rag layer volume of the vessel.

(3) FIG. 3 is a view of preferred embodiment of a vertically oriented dehydrator or separator vessel having a high voltage electrode grid in the oil layer of the vessel and a low voltage electrode grid in the interface emulsion or rag layer of the vessel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) A preferred embodiment of a system 10 made according to this invention includes a separator vessel 12 which may be of a horizontal or vertical type. For example, a NATCO DUAL POLARITY or DUAL FREQUENCY or PETRECO BILECTRIC Electrostatic Treater (Cameron Process Systems, Houston, Tex.) could be used as the vessel 12.

(5) A crude oil stream 22 containing entrained gas, water, and solid contaminants enters vessel 12 through an inlet 14. Vessel 12 holds and treats those components so that the oil might separate from the contaminants. The separated oil is then removed from vessel 12 through an outlet 20.

(6) During the separation process, it is common for oil-coated solids, called mud, to accumulate in a bottom portion of vessel 12 and for a layer comprising a mixture of oil and water, called interface emulsion or rag, to form at the oil-water interface 15 in an intermediate portion 17 of vessel 12. The water accumulates in a water volume 19 located between the layer of solids and the water-emulsion boundary layer 16 of the interface emulsion The gas contained in the upper portion of vessel 12 enters an outlet 18 and travels along path 24 for further processing, thereby eliminating the need to vent the gas contained in vessel 12. The oil accumulates above the interface emulsion in an oil layer volume 13 of the vessel, and the gas, in turn, typically accumulates above the oil in an upper portion of vessel 12.

(7) As shown in FIG. 2, a separator vessel 12 includes an electrode grid 30 in the oil layer volume 13 of the separator vessel 12. The electrode grid 30 is a high voltage grid in communication with a high voltage transformer 31 and, preferably, power electronics 33 to produce a variable amplitude and variable frequency voltage supply. Dual- or multiple-frequency systems and techniques like that disclosed in U.S. Pat. No. 7,351,320 B2 to Sams, which is hereby incorporated by reference, may be used to produce the variable amplitude and frequency. Preferably, the voltage of electrode grid 30 ranges from 1 to 60 kV. The electrode grid 30 may include a single pair of electrodes or multiple pairs of electrodes. A medium frequency transformer 35 may be provided for increased secondary voltage known to promote effective coalescence. This secondary voltage can be rectified so that polarized voltages can be applied to the electrode grid 30 to create the benefits of both AC and DC fields within vessel 12.

(8) To promote water coalescence in the interface emulsion or rag layer volume 17 of separator vessel 12, and therefore control the build-up of that layer, a second electrode grid 40 is located in the rag layer volume 17. The electrode grid 40 is in communication with a low voltage transformer 41 and, preferably, power electronics 43 that produce a variable amplitude and variable frequency voltage supply. Dual- or multiple-frequency systems and techniques like that disclosed in U.S. Pat. No. 7,351,320 B2 to Sams, which is hereby incorporated by reference, may be used to produce the variable amplitude and frequency. In some applications, two or more transformers 41, 43 may be used.

(9) The electrode grid 40 may include a single pair of electrodes or multiple pairs of electrodes. Preferably, the voltage is no greater than 5 kV. The resulting electrostatic field promotes coalescence of the water droplets within the interface emulsion layer, thereby reducing the volume of this layer and increasing the effective residence time within vessel 12 and the performance of vessel 12.

Experimental Results

(10) An apparatus was developed to determine electrostatic field effects on rag layer volume reduction. The apparatus was a small-scale flow-through unit consisting of a chamber where voltage and temperature can be applied ranging from 1 to 5 kV, and 80 F. to 300 F., respectively. A rag layer feed sample obtained from a commercial separator was utilized for the analysis.

(11) The experimental analysis was designed to determine the effects of applying the electrode grid in the rag layer at elevated operating temperature and pressure. Treated samples of the rag layer were collected for analysis of separation performance at two operating temperatures. The voltage remained reasonably constant at 1 kV when applied for each temperature. The separation performance was evaluated by centrifugal analysis (ASTM D4007 method) and gravitational separation rate at 5-minute intervals. Samples treated with low voltage are indicated as Treated below. Untreated samples did not have voltage applied and were utilized as a control. An Untreated rag sample showed no signs of water separation after it was permitted to settle for 30 minutes.

(12) The separation performance for the Treated samples is summarized in the following table:

(13) TABLE-US-00001 Treated Treated 240 F. 280 F. Volume (mL) Centrifugal Analysis Total 78.0 88.0 Rag 28.0 34.0 Water 43.4 52.5 Solid 6.6 1.5 Gravity Settling Analysis Time (min) (mL of free water) 5 62 20 10 80 30 15 90 35 20 90 50 25 90 50 30 90 50

(14) Analytical results show an appreciable difference between the Treated and Untreated samples. The results of the Untreated sample are indicative of a highly stabilized emulsion. In particular, all of the water in the Untreated sample existed as rag, and no separation of free water occurred when the sample was rested for 30 minutes. After applying voltage, the Treated samples showed separation of free water and solids as well as a reduction in rag volume indicating destabilization. In particular, the Treated samples show increased free water separation in both centrifugal and gravitational analysis. The centrifugal analysis for the Treated samples also indicates an increase in solids release and a reduction in the volume of the rag layer. Increased temperature did not increase separation performance.

(15) While the invention has been described with a certain degree of particularity, many changes could be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. The invention is not limited to the preferred embodiments described herein. Instead, the invention is limited to the scope of the attached claims, including the full range of equivalency to which each element thereof is entitled.